1. Field of the Invention
The present invention relates to component shielding. More specifically, the present invention relates to an electronic isolation shield for mounting to a circuit board, around an electronic component mounted thereon, and a method for determining the number of securing points required on the shield to sufficiently secure the shield to the circuit board.
2. Discussion of the Related Art
Isolation of certain components from sources of electromagnetic interference (EMI) is particularly important in communications equipment, such as, for example, cellular telephones and related relay station equipment, which have components that operate in at least the 50 MHz to 4 GHz frequency range. Conventional radio frequency (rf) isolation shields for electronic components do not provide sufficient shielding (i.e., greater than 60 dBm) without requiring the use of a large number of screws and a gasket to secure the shield to the component. More than one screw for each linear inch along the perimeter of the shield may be required at 1000 MHz to sufficiently ground and stably secure the shield walls to the planar surface of the electronic component (e.g., a printed circuit or p.c. board). Additionally, gaskets tend to leak when they are installed because of, inter alia, mechanical damage to the thin gasket material, an improper placement of the gasket material during assembly, or damage to the gasket material caused by heat, all of which cause EMI leakage. Furthermore, a new gasket may be needed each time the shield is reassembled, thereby increasing the costs associated with using a conventional shield.
Other attempts at providing a shield include the use of a cap that is attached and grounded by soldering. But soldering is expensive and may be less reliable than a screw and gasket mounting, especially if the cap has been removed and reattached. The reattached solder cap is generally less reliable because air gaps and other surface irregularities that typically form when the solder around the cap is removed may remain after the cap is re-soldered. Moreover, these caps may be difficult to unsolder and the unsoldering operation itself is risky, possibly causing heat damage to the electronic components or delaminating portions of the circuit board assembly.
U.S. Pat. No. 5,557,063, which includes a common inventor with the present application and is commonly assigned, is directed towards an electronic isolation shield that has a tongue on a sealing edge wall of the shield and a corresponding groove in the surface of the electronic component to provide effective shielding without the use a gasket. The disclosure of U.S. Pat. No. 5,557,063 is hereby incorporated by reference. The shield may be attached to the circuit board assembly using either screws or a snap fit as illustrated in FIGS. 3A and 3B of the '063 patent. However, we have discovered that, due to the effects of creep, the holding force of the resilient tabs in the snap fit embodiment decreases over time. The effects of creep are particularly pronounced when the shield is exposed to an environment of elevated temperatures, which is typical for much of its service life. The typical elevated temperature ranges from above room temperature to about 75.degree. C. Additionally, when attaching the shield onto the circuit board assembly, the tabs should not be overstressed nor should undue stress be applied to the electronic component assembly.